The major goals of the proposed research are to investigate the physiological mechanisms underlying the display of a hormone-facilitated behavior in the neonate and to trace their development. These studies are proposed in light of my recent finding that under certain conditions infant rats show components of receptive and proceptive behavior to tactile stimulation of the flanks. Moreover, these responsess are facilitated by exogeneous steroids during the first postanatal week when endogenous steroids are presumably sensitizing the neural circuitry underlying sexually dimorphic behavior. The proposed experiments first, determine the site and mechanism of estrogen activation of precocious reproductive function and second, describe the changes that occur as the responses become more steroid dependent, as sex differences in steroid sensitivity emerge, and as the eliciting stimuli become more specified. This analysis should provide new insights into the functional development of the neural systems mediating female sexual behavior and the process of sexual differentiation of brain and behavior.